When I have done this in the past I have always sat down with each user, had them login on their workgroup profile, show me where they were secreting all their files, then copied all their files to the new server, then logged out and logged in as their domain userID and setup shortcuts and drive maps to their files then copied any specialty shortcuts from their old desktop profile to the new logged in one.
I don't give them access to their old files on their local PCs. They only get access to their old files off server shares. I have told them "the new system doesn't support saving files locally" even though that is a lie. I don't delete their old profile because invariably I'll get a call a week later from some user who is hell-bent-convinced that some critical file of theirs was accidentally deleted when it was moved to the server. So I can have them logout, then login to their old workgroup profile and show me where the critical file is, then logout and log back in to their domain profile and show them were the file was moved to.
Most users who are older have had at least 1 disk crash in their lives so are more than happy to comply once they understand the reasoning why we save everything to the server and become more comfortable with doing it. I don't even bother with roaming profiles or any of that to force the issue, education is generally enough. It also gives me a chance to ID where they are saving all their web browser shortcuts and explain how browser sync works.
I've only had 1 user ever object to this, her thing was "I have HR files and it's safer for them to be on my workstation than the server" I didn't even bother arguing with her since she ended up retiring and the replacement that they hired for her understood the importance of saving everything on the server where it would be backed up.
